The invention relates to a braking system for motor vehicles as defined hereinafter. A dual-circuit braking system of this type is known from British Patent GB-PS 2 140 112 B.
A condition underlying the invention, mentioned in ECE Rule 13 and in specification 71/320/EWG, is that the design of the front-wheel brakes and the rear-wheel brakes must be selected such that the front wheels must block before the rear wheels, even with an empty vehicle, up to a vehicle deceleration of 0.82 G. It is intended by this that the vehicle remains directionally stable with increasing deceleration until blockage occurs, i.e., does not skid. When an automatic anti-locking device (ABS) is installed, it also applies with ABS valves in the initial position, for example, in case of a failure of the electrical system or a defect in the ABS control device, for instance.
In GB 2 140 112 B, which was mentioned at the outset, it is disclosed as known to improve the utilization of the braking force of the rear axle brakes by using so-called brake pressure regulators that control the brake pressure for the rear axle, dependent upon pressure, deceleration or load, as a function of the brake pressure on the front axle. Because this known design operates relatively inaccurately, however, GB 2 140 112 B proposes to execute a variable brake adjustment between the front and rear wheels, without additional brake pressure regulators, by means of the magnet valves of the anti-locking system that are present anyway. Such a braking adjustment permits a better utilization of the rear-axle brakes in the partial braking region, and uniform wear of the front- and rear-wheel brakes is thereby attained.
In this case pressure transducers are used as actual value transmitters, and quadripoles with transmission functions and comparators are used. Such pressure transducers are relatively expensive, yet they have zero-point defects and may produce varying ratios between charging pressure and electrical output voltage. The quadripoles also can have tolerances such that different front-wheel brake pressures result on the right and left. As an example, the characteristic curves of two quadripoles can have unequal curves.